1. Field of the Invention
The present invention relates to methods for degrading or converting plant cell wall polysaccharides and to products obtained by such methods.
2. Description of the Related Art
Plant cell walls are composed of a mixture of polysaccharides interlocked in a complex structure (Carpita et al., 2001, Plant Physiology 127: 551-565). The mixture of polysaccharides include cellulose, xyloglycan (hemicellulose), and pectic polymers, which are primarily composed of hexoses, e.g., glucose, galactose, and mannose; pentoses, e.g., xylose and arabinose; uronic acids, e.g., galacturonic acid and glucuronic acid; and deoxyhexoses, e.g., rhamnose and fucose.
Plant cell wall polysaccharides can be enzymatically degraded to glucose, xylose, mannose, galactose, and arabinose, which can then be converted to other organic substances, for example, glucose is easily fermented by yeast into ethanol. Wood, agricultural residues, herbaceous crops, and municipal solid wastes can be used as sources of plant cell wall polysaccharides.
Cellulose is a primary component of plant cell walls. Many microorganisms produce enzymes that degrade cellulose. These enzymes include, for example, endoglucanases, cellobiohydrolases, and beta-glucosidases. Endoglucanases digest the cellulose polymer at random locations, opening it to attack by cellobiohydrolases. Cellobiohydrolases sequentially release molecules of cellobiose from the ends of the cellulose polymer. Cellobiose is a water-soluble beta-1,4-linked dimer of glucose. Beta-glucosidases hydrolyze cellobiose to glucose.
Natural microorganisms that degrade cellulose and other cell wall polysaccharides may not be ideal for large-scale conversion of cellulosic materials because (a) the full complement of enzymes may be lacking, (b) one or more enzyme components perform poorly, are labile, or their kinetic behavior fails to meet the specification of the intended use, (c) the conversion and/or degradation could be improved by expression of a heterologous enzyme gene that enhances the conversion/degradation, or (d) the full complement of enzymes may be in insufficient amounts to be economically viable. It would be an advantage to the art to improve the degradation and conversion of plant cell wall polysaccharides by using whole fermentation broth from recombinant microorganisms to circumvent expensive cell removal and enzyme formulation steps.
It is an object of the present invention to provide new methods for degrading or converting plant cell wall polysaccharides into various products using spent whole fermentation broths from recombinant microorganisms.